1. Field of the Invention
The subject invention relates to lasers and more particularly, to a method for providing optical elements of effectively tapered reflectivity useful in restricting the mode of operation of relatively long wavelength waveguide lasers.
2. Description of Related Art
In certain applications, such as material processing, laser radar, phased arrays and missile guidance, it is desirable to have a laser whose output is restricted to a particular mode, for example, the EH.sub.11 mode. Where a laser is turned on and operated for a short period, its performance may be dominated by warm-up or thermal equilibration effects. During warm-up, for example, the length of the laser cavity changes, and the laser output oscillates through a series of modes.
In the prior art, mode control is typically obtained by careful alignment of the cavity optics. The lasers do not usually have adequate mode control to ensure operation in the preferred EH.sub.11 mode over the full operating range. Additional EH.sub.11 mode control may be achieved by decreasing the waveguide bore size and/or increasing the dielectric losses at the waveguide walls to increase waveguide losses for higher modes. Reducing the waveguide bore size is generally successful in controlling mode but results in a substantial reduction in laser output power and efficiency. Increasing the dielectric losses (e.g. by roughing the waveguide walls) is not reproducible and usually does not provide adequate mode control.
As disclosed in the article "Formation of non-Gaussian light beams in a laser with an exit mirror characterized by a smooth amplitude inhomogeneity," Soviet Journal of Quantum Electronics, July 1986, mirrors with a smooth variation in reflectivity have been used on non-waveguide lasers to achieve mode control or to force operation on a higher order Gaussian mode. To date, there is no reported application of this approach to waveguide lasers. Fabrication of such mirrors, which is achieved by an evaporation process, is expensive. Such a procedure must be carried out in a manufacturing or laboratory environment, typically using a vacuum deposition system to treat a large number of optic elements. It is thus not suitable for quick, inexpensive field modification of existing lasers. It is also not easily adaptable to applications where the optic (mirror) is not aligned with the center of the bore hole of the laser.